1. Field of the Invention
The present invention relates to an optical system and an image taking apparatus, and more particularly to an optical system and an image taking apparatus capable of forming an image of the object utilizing an optical element with one or plural internal reflecting surfaces and adapted for use in a video camera, a still video camera, an observation apparatus or the like.
2. Related Background Art
There have already been proposed various image taking optical systems and observation optical systems based on a refractive system. For improving the imaging performance, these optical systems are well corrected for aberrations such as the spherical aberration, coma aberration, imaging plane curvature etc. for the reference wavelength, and are corrected for the chromatic aberration (so-called achromatic) in the visible wavelength range.
In the optical system utilizing ordinary lenses, it is basically impossible to achieve achromatic property with a single lens, so that the correction of chromatic aberration is realized by the combination of plural lenses of mutually different dispersions.
On the other hand, there have also been proposed various image taking optical systems utilizing a reflecting surface such as a concave mirror or a convex mirror. Since such reflecting surface does not in principle generate the chromatic aberration, such optical systems are widely utilized in telescopes in which the imaging performance is significantly affected by the chromatic aberration. FIG. 17 is a schematic view of a mirror optical system composed of a concave mirror and a convex mirror.
In the mirror optical system shown in FIG. 17, a light beam 104 from an object is reflected by a concave mirror 101, thus being directed toward the object in a gradually converging state, then reflected by a convex mirror 102 and is focused on an imaging plane 103.
The optical system shown in FIG. 17 constitutes the basic configuration of a so-called Cassegrain reflective telescope, in which a telescopic optical system with a large entire length composed of ordinary lenses is folded up with two mutually opposed mirrors to reduce the entire length of the optical system, and the generation of chromatic aberration, inherent to the telescopic lens, is avoided by the use of reflective mirrors.
In this manner the replacement of the lenses with the mirrors allows to efficiently fold back the optical path and to obtain a compact optical system without the influence of chromatic aberration, but it is difficult, for a cataptoric optical system consisting solely of mirrors, to satisfactorily correct all the aberrations of the entire system.
For this reason, there are also known optical systems employing a mirror system and a lens system in combination for increasing the freedom of aberration correction, and capable of correcting the aberrations in the entire system by a balanced combination of such mirror system and lens system. FIG. 18 shows an example of a catadioptoric optical system employing the combination of a mirror system and lens system. Referring to FIG. 18, a light beam 116 from an object is refracted by lenses 111, 112, then reflected by a concave mirror 113, thus being directed toward the object in a gradually converging state, then reflected by a convex mirror 114 and focused on an imaging plane 115. The lens system is so designed as to cancel the aberrations generated by the mirrors.
However, the lens system is composed of the combination of a convex lens 111 and a concave lens 112 in order to correct the chromatic aberration. Therefore, though the optical system is made compact by the efficient folding of the optical path by the mirror system alone, the entire optical system becomes bulky, requiring lenses of a large diameter. Also as the number of optical components becomes larger, there is required a precise assembling operation for these optical components. In particular it is essential to precisely adjust the position and angle of each mirror, since strict relative positional precision is required between the mirrors and for each mirror relative to the lenses.
For avoiding such drawback, there has been proposed, for example in the Japanese Patent Laid-open Application No. 8-292371, to form the mirror system, or the mirror system and the lens system, as a single block thereby avoiding the assembling error of the optical components in the assembling operation.
On the other hand, there are already known optical components having plural reflective surfaces on the surface of a single block, for example optical prisms such as a polo prism or a pentagonal roof prism employed in the view finder optical system.
These prisms, having plural reflecting surfaces in an integral manner with a precise positional relationship, do not require the mutual positional adjustment of the reflecting surfaces. However, these prisms are intended to invert the image by varying the advancing direction of the light, and the reflecting surfaces are composed of flat planes.
On the other hand, there are also known prism optical systems having curved reflecting surfaces.
FIG. 19 is a schematic view of an observation optical system disclosed in the U.S. Pat. No. 4,775,217. This observation optical system is designed to observe an external scene and also to observe an image displayed on an information display member, in an overlapping manner with the scene.
In this observation optical system, a light beam 125 emerging from a display surface of the information display member 121 is directed to the object by reflection by a surface 122 and is incident on a concave half mirror 123. After being reflected by the half mirror surface 123, the display light beam 125 becomes a substantially parallel light beam by the refractive force of the concave surface 123, then is transmitted and refracted by surface 122 and is incident on the pupil 124 of the observer, thereby causing the observer to recognize an enlarged false image of the displayed image.
On the other hand, a light beam 126 from an object is incident on a surface 127 approximately parallel to the reflective surface 122, is refracted by the surface 127 and is incident on the concave half mirror 123. A part of the object light beam 126 is transmitted by the concave surface 123, bearing an evaporated half-transmitting film, then transmitted and refracted by the surface 122 and is incident on the pupil 124 of the observer. Consequently the observer observes the displayed image overlapping with the external scene.
FIG. 20 is a schematic view of an observation optical system disclosed in the Japanese Patent Laid-open Application No. 2-297516. This optical system is also designed to observe the external scene and also to observe an image, displayed on an information display member, in an overlapping manner.
In this observation optical system, a light beam 134 emerging from an information display member 130 is transmitted by a flat plane 137 constituting a prism Pa, thereby being incident on the prism Pa, and is incident on a parabolic reflective surface 131. Being reflected by the parabolic reflective surface 131, the display light beam 134 becomes a converting light beam focusing on a focal plane 136. The display light beam 134, reflected by the reflective surface 131, reaches the focal plane 136 by repeating total reflections by two parallel flat surfaces 137, 138 constituting the prism Pa, whereby the entire optical system can be made thinner.
The display light beam 134 emerging from the focal plane 136 in diverging state repeats total reflections on the flat surfaces 137, 138 and is incident on a parabolic half mirror 132. It is thus reflected by the half mirror 132 and is converted into a substantially parallel light beam by the refractive force thereof, thereby forming an enlarged false image of the displayed image. It is then transmitted by a surface 137 and is incident on the pupil 133 of the observer, whereby the observer can observe the displayed image.
On the other hand, a light beam 135 from an external object is transmitted by a surface 138b constituting a prism Pb, then transmitted by a parabolic half mirror 132 and the surface 137 and is incident on the pupil 133 of the observer. Consequently the observer can observe the displayed image in an the external scene in overlapping manner.
However, in the above-explained optical system in which plural reflective surfaces are formed in a single block, no particular correction of the aberrations is intended, and the chromatic aberration generated at the entrance and exit surfaces becomes a problem because such block is constituted by a dispersing medium such as glass.
Furthermore, an optical head for the optical pickup, disclosed for example in the Japanese Patent Laid-open Application Nos. 5-12704 and 6-139612, reflects the light from a semiconductor laser by a Fresnel surface or a hologram surface formed on a prism, then focuses the light onto a disk surface and guides the light reflected from the disk surface to a detector. Such optical system for the optical pickup, being designed for the laser light, has an extremely narrow wavelength range, and the correction of the chromatic aberration over the visible wavelength range, as in the image taking optical system, is not contemplated at all.